The combination of spatial processing using beamforming techniques (i.e., multiple-microphones) and binaural listening is applicable to a variety of fields and is particularly applicable to the hearing aid industry. This combination offers the benefits associated with spatial processing, i.e., noise reduction, with those associated with binaural listening, i.e., sound location capability and improved speech intelligibility.
Beamforming techniques, typically utilizing multiple microphones, exploit the spatial differences between the target speech and the noise. In general, there are two types of beamforming systems. The first type of beamforming system is fixed, thus requiring that the processing parameters remain unchanged during system operation. As a result of using unchanging processing parameters, if the source of the noise varies, for example due to movement, the system performance is significantly degraded. The second type of beamforming system, adaptive beamforming, overcomes this problem by tracking the moving or varying noise source, for example through the use of a phased array of microphones.
Binaural processing uses binaural cues to achieve both sound localization capability and speech intelligibility. In general, binaural processing techniques use interaural time difference (ITD) and interaural level difference (ILD) as the binaural cues, these cues obtained, for example, by combining the signals from two different microphones.
Fixed binaural beamforming systems and adaptive binaural beamforming systems have been developed that combine beamforming with binaural processing, thereby preserving the binaural cues while providing noise reduction. Of these systems, the adaptive binaural beamforming systems offer the best performance potential, although they are also the most difficult to implement. In one such adaptive binaural beamforming system disclosed by D. P. Welker et al., the frequency spectrum is divided into two portions with the low frequency portion of the spectrum being devoted to binaural processing and the high frequency portion being devoted to adaptive array processing. (Microphone-array Hearing Aids with Binaural Output-part II: a Two-Microphone Adaptive System, IEEE Trans. on Speech and Audio Processing, Vol. 5, No. 6, 1997, 543–551).
In an alternate adaptive binaural beamforming system disclosed in co-pending U.S. patent application Ser. No. 09/593,728, filed Jun. 13, 2000, two distinct adaptive spatial processing filters are employed. These two adaptive spatial processing filters have the same reference signal from two ear microphones but have different primary signals corresponding to the right ear microphone signal and the left ear microphone signal. Additionally, these two adaptive spatial processing filters have the same structure and use the same adaptive algorithm, thus achieved reduced system complexity. The performance of this system is still limited, however, by the use of only two microphones.